This invention relates to commutation of the stator circuit of brushless DC motors using magnetically actuated switches responsive to the field from a rotating permament magnet on the rotor shaft to control the conductivity of semiconductor means connected to stator coils to start and run the motor.
In my U.S. Pat. No. 3,569,806, a two-transistor bistable commutator circuit is connected to the stator coils of a brushless DC motor to control current through the coils when the motor is running. The starting circuit uses illumination from an incandescent light source to activate one or the other of two photoconductive devices connected to the bases of the two transistors. A shutter attached to the rotor to rotate with it allows light from the source to reach only one of the photoconductive devices at a time. When power is applied to start the motor, the photoconductive device that is initially illuminated depends on the position at which the rotor and shutter had come to a stop. Each photoconductive device is connected to one of the transistors, and the transistor to which the photoconductive device that is first illuminated is connected is the transistor that must become conductive to cause starting current to flow in the correct stator coil or coils. Stator flux from this coil, or set of coils, causes the rotor to start turning in the correct direction.
Bistable circuits normally have two states of conductivity and remain in either of those states indefinitely until triggered into the other state by a suitable signal. In U.S. Pat. No. 3,569,806, the triggering voltage is initially derived from the two photoconductive devices, but once the motor is running, the light source may be turned off. Thereafter, the triggering signals to cause reversal of the conductivity of the two transistors are obtained across the respective sets of stator coils. Such voltages are produced as long as the rotor continues to turn, but if it is stopped by an excessive mechanical load, it may not start again unless the light source is re-energized.
In my co-pending application Ser. No. 918,711 entitled BRUSHLESS MOTOR CONTROLLED BY RADIANT ENERGY, two sets of stator coils are provided. Each of these sets is connected in series with the emitter-collector output circuit of its own transistor. The transistors are not connected to each other to apply actuating signals from one set of stator coils to the other. As a result, the two sides of the commutating circuit are electrically independent of each other, and each is actuated by electro-optical means consisting of separate light-emitting diodes (L.E.D's) positioned to direct light to an adjacent photoconductive diode. The photoconductive diodes are connected to the respective bases of the commutating transistors, and light from each of the L.E.D's is controlled by an episcotister so that only one of the photoconductive diodes at a time receives light from its respective L.E.D. The photoconductive diode thus energized at any given instant is the one that must be energized to cause the transistor to which it is connected to be conductive in order to draw current through the proper stator coil, or coils, to cause the rotor to start rotating or to continue to rotate, as required.
In the circuit just described, each of the photoconductive diodes acts like a single-pole, single-throw switch. It is only because the photoconductive diodes receive light by way of the same episcotister that they cooperate in their operation so that they are, in effect, a single-pole double-throw switch that supplies actuating signals alternately to the bases of the two commutating transistors. Since this circuit does not depend on feedback from one transistor to the other, and since the L.E.D's need not be turned on because they draw so little current that they do not substantially reduce the life of a battery that operates the motor, the motor is always in condition to start, even if it is momentarily stopped by an excessive load.